Open-loop charge pump for increasing ripple frequency of output voltage

ABSTRACT

An open-loop charge pump is provided. In the open-loop charge pump, a peak current limiting control circuit is arranged between a control circuit and a boost circuit. The control circuit drives the peak current limiting control circuit based on an over-voltage protection signal to control the boost circuit to be in a charging phase continuously or in a normal operation mode. In a case that the output voltage is higher than an upper threshold voltage, the boost circuit is continuously in the charging phase and does not supply power to an output load, and an output capacitor is discharged to supply power to the output load. In a case that the output voltage is lower than a lower threshold voltage, the boost circuit is in the normal operation mode.

This application claims the priority to Chinese Patent Application No.201810038898.X, titled “OPEN-LOOP CHARGE PUMP”, filed on Jan. 16, 2018with the Chinese Patent Office, which is incorporated herein byreference in its entirety.

FIELD

The present disclosure relates to the technical field of integratedcircuits, and in particular to an open-loop charge pump.

BACKGROUND

A charge bump, which is also referred to as a non-inductive directcurrent/direct current (DC/DC) converter, is a switch DC/DC converter inwhich a capacitor is used for power storage. In a portable electronicdevice powered by a battery, a boost charge pump circuit is often usedas a power supply converter. Compared with an inductive DC/DC converterin which an inductor is used for power storage, the charge-pump typeconverter with no power inductor has smaller electromagneticinterference (EMI), occupies a smaller area on the board, and has lowersystem cost. The charge pumps are generally divided intovoltage-stabilizing output type charge pumps and non-voltage-stabilizingoutput type charge pumps. In the voltage-stabilizing output type chargepump circuit, a close-loop control structure is usually adopted in whichan output voltage of the charge pump circuit is sampled by using adividing resistor to obtain a sampled voltage. The sampled voltage iscompared with a preset standard voltage, and the output voltage isstabilized to a preset value by using a switch transistor forcontrolling the power of the charge pump in an output control circuit ofan error amplifier. The switch transistor cannot be fully turned onunder a gate voltage of the switch transistor, and the conductionimpedance is not minimum, which affects the driving capability of thecharge pump. Further, since a feedback loop exists in the close-loopcharge pump, it is required to consider stability of the loop in thecharge pump. In addition, the error amplifier is included in theclose-loop charge pump, which results in large power consumption of theclose-loop charge pump and a large area of an integrated chip.

In the open-loop charge pump, a gate voltage of the switch transistorcan reach a power supply voltage and a ground voltage, and theconduction impedance can be minimum, while it is not required to performloop control. Therefore, the open-loop charge pump is widely applied dueto high efficiency and driving capability. The open-loop charge pumpgenerally has a current limiting function and an over-voltage protectionfunction, which ensures effective and reliable operation of theopen-loop charge pump. During the operation of the open-loop chargepump, when the power supply voltage exceeds an over-voltage protectionentering threshold for the charge pump, the open-loop charge pump entersan over-voltage protection mode, and a load is powered by an outputcapacitor. If a current of the load is small, it takes a long time forthe output voltage to be decreased to an over-voltage protection exitingthreshold. In this case, a ripple frequency of the output voltage is lowand may fall in the audio frequency range (from 20 Hz to 20 KHz), whichinterferes an audio signal and generates audible noise.

SUMMARY

An open-loop charge pump is provided in the present disclosure, whichcan increase a ripple frequency of an output voltage and remove musicnoise of the open-loop charge pump, thereby improving audio quality.

The open-loop charge pump provided in the present disclosure includes: acontrol circuit, a boost circuit, an output voltage detection circuit,an over-voltage protection circuit, a peak current limiting controlcircuit, and a clock circuit. The control circuit includes a first inputterminal and an output terminal. The boost circuit includes a controlterminal and a load terminal. The load terminal of the boost circuit isconnected to a load circuit including an output capacitor and an outputload, and the load terminal of the boost circuit is connected to thefirst input terminal of the control circuit via the output voltagedetection circuit and the over-voltage protection circuit. The outputvoltage detection circuit is configured to output a detection voltagebased on an output voltage at the load terminal, and the over-voltageprotection circuit is configured to output an over-voltage protectionsignal based on a reference voltage and the detection voltage. Theoutput terminal of the control circuit is connected to the controlterminal of the boost circuit via the peak current limiting controlcircuit. The control circuit drives the peak current limiting controlcircuit based on the over-voltage protection signal to control the boostcircuit to be in a charging phase continuously or in a normal operationmode. In a case that the output voltage is higher than an upperthreshold voltage, the boost circuit is continuously in the chargingphase and does not supply power to the output load, and the outputcapacitor is discharged to supply power to the output load. In a casethat the output voltage is lower than a lower threshold voltage, theboost circuit is in the normal operation mode. In a time period of thenormal operation mode, the boost circuit switches between a dischargingphase and the charging phase based on a frequency of the clock circuit.When the boost circuit switches from the charging phase to thedischarging phase, the peak current limiting control circuit decreases apeak current outputted from the boost circuit to increase a ripplefrequency of the output voltage.

In an embodiment, the peak current limiting control circuit includes: afirst switch transistor, a second switch transistor, a third switchtransistor and a single pulse generator. A gate of the first switchtransistor is connected to the control terminal of the boost circuit, afirst electrode of the first switch transistor is configured to receivea power supply voltage, and a second electrode of the first switchtransistor is connected to the gate of the first switch transistor. Agate of the second switch transistor is connected to an output terminalof the single pulse generator, a first electrode of the second switchtransistor is grounded, and a second electrode of the second switchtransistor is connected to the second electrode of the first switchtransistor. A gate of the third switch transistor is connected to theoutput terminal of the single pulse generator via a phase inverter, afirst electrode of the third switch transistor is grounded, and a secondelectrode of the third switch transistor is connected to the secondelectrode of the first switch transistor via a current source. An inputterminal of the phase inverter is connected to the output terminal ofthe single pulse generator, and an output terminal of the phase inverteris connected to the gate of the third switch transistor. An inputterminal of the single pulse generator is connected to the outputterminal of the control circuit.

In an embodiment, the first switch transistor is a P-channel Metal OxideSemiconductor (PMOS) transistor, and both the second switch transistorand the third switch transistor are N-channel Metal Oxide Semiconductor(NMOS) transistors.

In an embodiment, the boost circuit includes a fourth switch transistorand a function circuit. A gate of the fourth switch transistor is thecontrol terminal of the boost circuit, a first electrode of the fourthswitch transistor is configured to receive the power supply voltage, anda second electrode of the fourth switch transistor is connected to theload circuit via the function circuit. The fourth switch transistor andthe first switch transistor form a current mirror to limit an outputcurrent of the second electrode of the fourth switch transistor, so asto decrease the peak current.

In an embodiment, the fourth switch transistor is a PMOS transistor.

In an embodiment, the over-voltage protection circuit outputs theover-voltage protection signal having a high level in a case that theoutput voltage is higher than the upper threshold voltage, and outputsthe over-voltage protection signal having a low level in a case that theoutput voltage is lower than the lower threshold voltage.

In an embodiment, in a case that the over-voltage protection circuitoutputs the over-voltage protection signal having a high level, thecontrol circuit controls, based on the over-voltage protection signal,the single pulse generator to output a positive pulse signal. Thepositive pulse signal is used to control the second switch transistor tobe turned on, and the positive pulse signal is converted to a negativepulse signal via the phase inverter. The negative pulse signal is usedto control the third switch transistor to be turned off, so as tocontrol the fourth switch transistor to be in the charging phase.

In an embodiment, in a case that the over-voltage protection circuitoutputs the over-voltage protection signal having a low level, thecontrol circuit controls, based on the over-voltage protection signal,the single pulse generator to output a negative pulse signal. Thenegative pulse signal is used to control the second switch transistor tobe turned off, and the negative pulse signal is converted to a positivepulse signal through via the phase inverter. The positive pulse signalis used to control the third switch transistor to be turned on such thatthe current source provides a current-limiting current for the currentmirror, so as to decrease the peak current.

In an embodiment, the single pulse generator is configured to output apulse signal having a signal width of 0.5 μs.

It can be seen from the above description that, in the open-loop chargepump provided according to the technical solution of the presentdisclosure, the peak current limiting control circuit is arrangedbetween the control circuit and the boost circuit. The control circuitdrives the peak current limiting control circuit based on theover-voltage protection signal to control the boost circuit to be in thecharging phase continuously or in the normal operation mode. In a casethat the output voltage is higher than the upper threshold voltage, theboost circuit is continuously in the charging phase and does not supplypower to the output load, and the output capacitor is discharged tosupply power to the output load. In a case that the output voltage islower than the lower threshold voltage, the boost circuit is in thenormal operation mode. In a time period of the normal operation mode,the boost circuit switches between the discharging phase and thecharging phase based on the frequency of the clock circuit. When theboost circuit switches from the charging phase to the discharging phase,the peak current limiting control circuit decreases the peak currentoutputted from the boost circuit to increase the ripple frequency of theoutput voltage. In this way, the music noise, which is caused by theripple frequency of the output voltage of the charge pump being in anaudio frequency range, can be avoided, thereby improving the audioquality.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate technical solutions in embodimentsof the present disclosure or in the conventional technology, thedrawings to be used in the description of the embodiments or theconventional technology are briefly described below. Apparently, thedrawings in the following description show only some embodiments of thepresent disclosure, and other drawings may be obtained by those skilledin the art from the drawings without any creative work.

FIG. 1 is a schematic circuit diagram of an open-loop charge pump;

FIG. 2 is a timing diagram illustrating a voltage of the open-loopcharge pump shown in FIG. 1;

FIG. 3 is a schematic circuit diagram of an open-loop charge pump forincreasing a ripple frequency of an output voltage according to anembodiment of the present disclosure;

FIG. 4 is a schematic circuit diagram of a peak current limiting controlcircuit in the open-loop charge pump shown in FIG. 3;

FIG. 5 is a schematic diagram showing waveforms of input and outputsignals of a single pulse generator; and

FIG. 6 is a timing diagram illustrating comparison between ripplefrequencies of output voltages of the open-loop charge pumps shown inFIG. 1 and FIG. 4.

DETAILED DESCRIPTION OF EMBODIMENTS

The technical solutions in the embodiments of the present disclosure aredescribed clearly and completely in conjunction with the drawings in theembodiments of the present disclosure hereinafter. It is apparent thatthe described embodiments are only some embodiments of the presentdisclosure, rather than all embodiments. Any other embodiments obtainedby those skilled in the art based on the embodiments of the presentdisclosure without any creative work fall within the protection scope ofthe present disclosure.

Reference is made to FIG. 1 and FIG. 2. FIG. 1 is a schematic circuitdiagram of an open-loop charge pump, and FIG. 2 is a timing diagramillustrating a voltage of the open-loop charge pump shown in FIG. 1. Theopen-loop charge pump includes: an output voltage detection circuit 14,an over-voltage protection circuit 13, a control circuit 12, a clockcircuit 16, a boost circuit 11, an over-current detection circuit 17 anda reference circuit 15. The boost circuit 11 provides an output voltageVOUT to a load circuit 10.

The output voltage detection circuit 14 generates a detection voltageVovp for the over-voltage protection circuit 13 by dividing the outputvoltage with resistors. The over-voltage protection circuit 13 comparesa reference voltage Vref with the detection voltage Vovp, and thengenerates an over-voltage protection signal OVP. The over-voltageprotection signal OVP is inputted to the control circuit 12. The controlcircuit 12 determines whether the boost circuit 11 is in a normaloperation mode or in an over-voltage protection mode based on theover-voltage protection signal OVP. The clock circuit 16 generates aswitch clock signal OSC for the control circuit 12 and the boost circuit11. Under a sequence condition corresponding to the clock signal OSC,the control circuit 12 generates a boost control signal Vcon based onthe over-voltage protection signal OVP. The boost control signal Vcon isused to control a switch transistor in the boost circuit 11 to be turnedon or turned off, so as to charge or discharge flying capacitors CF1 andCF2 in the open-loop charge pump and generate the output voltage VOUT.

In the open-loop charge pump shown in FIG. 1, since no closed loop isprovided for controlling the output voltage, the over-voltage protectioncircuit 13 and the output voltage detection circuit 14 are required todetect the output voltage VOUT. When the output voltage VOUT exceeds anupper threshold voltage V_(OVIPN), the over-voltage protection circuit13 controls the open-loop charge pump to enter the over-voltageprotection mode and generates the over-voltage protection signal OVPhaving a high level. The over-voltage protection signal OVP is inputtedto the control circuit 12 such that the control circuit 12 generates theboost control signal Vcon. The switch transistor in the boost circuit 11is in a charging phase under control of the boost control signal Vcon,and does not supply power to an output load Rout. In this case, theoutput load Rout is powered by an output capacitor Cout, and the outputvoltage VOUT of the boost circuit 11 is gradually decreased. When theoutput voltage VOUT is decreased to a lower threshold voltageV_(OVPOUT), the over-voltage protection signal OVP changes to a lowlevel, and the open-loop charge pump operates normally, and the outputvoltage VOUT is increased.

In the open-loop charge pump shown in FIG. 1, since a transient peakcurrent of the switch transistor of the boost circuit 11 is uncontrolledwhen the open-loop charge pump exits the over-voltage protection mode, alarge peak current completely flows to the output capacitor Cout. Inthis case, a ripple amplitude of the output voltage of the open-loopcharge pump is increased, which results in a decreased ripple frequencyof the output voltage, music noise in the audio frequency range, andpoor audio quality.

The open-loop charge pump shown in FIG. 1 has the followingdisadvantages. In a case that the open-loop charge pump is in theover-voltage protection mode, the switch transistor in the boost circuit11 is in the charging phase or is turned off, and the output load Routis powered by the output capacitor Cout. Further, when the open-loopcharge pump exits the over-voltage protection mode, a large transientpeak current IPEAK of the switch transistor, which is generally up to 2A, completely flows to the output capacitor Cout, the output voltageVOUT is increased, and the highest voltage of the output voltage VOUT isincreased, and thus the ripple amplitude of the output voltage VOUT isincreased. In this case, the over-voltage protection circuit 13 outputsthe over-voltage protection signal OVP to control the open-loop chargepump to be in the over-voltage protection mode. As shown in FIG. 2, as acurrent in the load circuit 10 become small, the output voltage VOUT isgradually decreased from a peak voltage, which results in the decreasedripple frequency of the output voltage, music noise in the audiofrequency range, and the poor audio quality.

In an embodiment of the present disclosure, a peak current limingcontrol module is added to the open-loop charge pump shown in FIG. 1, tosolve the above problem.

In order to make the above objects, features and advantages of thepresent disclosure more clear, the present disclosure is furtherdescribed in detail in conjunction with the drawings and specificembodiments.

Reference is made to FIG. 3, which is a schematic circuit diagram of anopen-loop charge pump for increasing a ripple frequency of an outputvoltage according to an embodiment of the present disclosure. Theopen-loop charge pump shown in FIG. 3 includes a control circuit 12 anda boost circuit 11. The control circuit 12 includes a first inputterminal and an output terminal. The boost circuit 11 includes a controlterminal and a load terminal.

The load terminal of the boost circuit 11 is connected to a load circuit10. The load circuit 10 includes an output capacitor Cout and an outputload Rout. The load terminal of the boost circuit 11 is connected to thefirst input terminal of the control circuit 12 via an output voltagedetection circuit 14 and an over-voltage protection circuit 13.

The output voltage detection circuit 14 is configured to output adetection voltage Vovp based on an output voltage VOUT at the loadterminal. The over-voltage protection circuit 13 is configured to outputan over-voltage protection signal OVP based on a reference voltage Vrefand the detection voltage Vovp.

The output terminal of the control circuit 12 is connected to thecontrol terminal of the boost circuit 11 via a peak current limitingcontrol circuit 18. The control circuit 12 drives the peak currentlimiting control circuit 18 based on the over-voltage protection signalOVP to control the boost circuit 11 to be in a charging phasecontinuously or in a normal operation mode.

In a case that the output voltage VOUT is higher than an upper thresholdvoltage V_(OPVIN), the boost circuit 11 is in the charging phase anddoes not supply power to the output load Rout, and the output capacitorCout is discharged to supply power to the output load Rout. In a casethat the output voltage VOUT is lower than a lower threshold voltageV_(OVPOUT), the boost circuit 11 is in the normal operation mode. In atime period of the normal operation mode, the boost circuit 11 switchesbetween a discharging phase and the charging phase based on a frequencyof a clock circuit. When the boost circuit 11 switches from the chargingphase to the discharging phase, the peak current limiting controlcircuit 18 is used to decrease a peak current outputted from the boostcircuit 11 to increase a ripple frequency of the output voltage VOUT.

In the open-loop charge pump shown in FIG. 3, the boost circuit 11further includes an over-current detection terminal, and the controlcircuit 12 further includes a second input terminal. The over-currentdetection terminal is connected to the second input terminal via anover-current detection circuit 17. The over-current detection circuit 17is used to detect a current of the boost circuit 11, generate a currentdetection signal OCP based on a current detection result, and transmitthe current detection signal OCP to the control circuit 12.

The control circuit 12 further includes a third input terminal. Thethird input terminal is connected to a clock circuit 16. The clockcircuit 16 generates a switch clock signal OSC for the control circuit12 and the boost circuit 11.

The control circuit 12 further includes a fourth input terminal. Thefourth input terminal is connected to a reference circuit 15. Thereference circuit 15 is used to provide the reference voltage Vref forthe over-voltage protection circuit 13.

In the embodiment shown in FIG. 3, the peak current limiting controlcircuit 18 is added to the open-loop charge pump shown in FIG. 1, whichsolves the problem of the poor audio quality caused by the extremely lowripple frequency of the output voltage VOUT. When the output voltageVOUT exceeds the upper threshold voltage V_(OVPIN), the generatedover-voltage protection signal OVP is a high level. The over-voltageprotection signal OVP is inputted to the control circuit 12 and the peakcurrent limiting control circuit 18, and the switch transistor in theboost circuit 11 is in the charging phase under control of a boostcontrol signal Vcon and does not supply power to the output load Rout.In this case, the output load Rout is powered by the output capacitorCout, and the output voltage VOUT is gradually decreased. When theoutput voltage VOUT is decreased to the lower threshold voltageV_(OVPOUT), the over-voltage protection signal OVP changes from the highlevel to a low level, and the peak current limiting control circuit 18generates a single pulse signal to perform current limiting control onthe switch transistor in the boost circuit 11, such that acurrent-limiting current Ilimit flows through the switch transistor inthe open-loop charge pump. In a time period of the single pulse signal,the current-limiting current Ilimit completely flows to the outputcapacitor Cout. In this way, the output voltage VOUT is increased in asmaller amplitude as compared with the case shown in FIG. 1, therebyincreasing the ripple frequency of the output voltage VOUT.

Reference is made to FIG. 4, which is a schematic circuit diagram of apeak current limiting control circuit in the open-loop charge pump shownin FIG. 3. The peak current limiting control circuit 18 includes: afirst switch transistor MPlim, a second switch transistor MN0, a thirdswitch transistor MN1, and a single pulse generator 181. In anembodiment, the single pulse generator 181 is configured to output apulse signal having a signal width of 0.5 μs. It should be noted thatthe signal width may be adjusted according to requirements, which is notlimited to 0.5 μs.

A gate of the first switch transistor MPlim is connected to the controlterminal of the boost circuit 11, a first electrode of the first switchtransistor MPlim is configured to receive a power supply voltage VBAT,and a second electrode of the first switch transistor MPlim is connectedto the gate of the first switch transistor MPlim. A gate of the secondswitch transistor MN0 is connected to an output terminal of the singlepulse generator 181, a first electrode of the second switch transistorMN0 is grounded, and a second electrode of the second switch transistorMN0 is connected to the second electrode of the first switch transistorMPlim. A gate of the third switch transistor MN1 is connected to theoutput terminal of the single pulse generator 181 via a phase inverter183, a first electrode of the third switch transistor MN1 is grounded,and a second electrode of the third switch transistor MN1 is connectedto the second electrode of the first switch transistor MPlim via acurrent source 182.

An input terminal of the phase inverter 183 is connected to the outputterminal of the single pulse generator 181, and an output terminal ofthe phase inverter 183 is connected to the gate of the third switchtransistor MN1. That is, a single pulse signal OVP_ILN inputted to thethird switch transistor MN1 and a single pulse signal OVP_IL inputted tothe second switch transistor MN0 have the same amplitude and oppositephases. An input terminal of the single pulse generator 181 is connectedto the output terminal of the control circuit 12.

In an embodiment, the first switch transistor MPlim is a PMOStransistor, and both the second switch transistor MN0 and the thirdswitch transistor MN1 are NMOS transistors.

The boost circuit 11 includes a fourth switch transistor MP0 and afunction circuit 111. A gate of the fourth switch transistor MP0 is thecontrol terminal of the boost circuit 11, a first electrode of thefourth switch transistor MP0 is configured to receive the power supplyvoltage VBAT, and a second electrode of the fourth switch transistor MP0is connected to the load circuit 10 via the function circuit 111. Theboost circuit 11 may be implemented by an existing boost circuit, whichis not described in detail herein.

The fourth switch transistor MP0 and the first switch transistor MPlimform a current mirror to limit an output current Ilimit of the secondelectrode of the fourth switch transistor MP0, so as to decrease thepeak current. In an embodiment, the fourth switch transistor MP0 is aPMOS transistor.

In the open-loop charge pump according to the embodiment of the presentdisclosure, the over-voltage protection circuit 13 outputs theover-voltage protection signal OVP having a high level in a case thatthe output voltage VOUT is higher than the upper threshold voltageV_(OVPIN), and outputs the over-voltage protection signal OVP having alow level in a case that the output voltage VOUT is lower than the lowerthreshold voltage V_(OVPOUT).

In the open-loop charge pump according to the embodiment of the presentdisclosure, in a case that the over-voltage protection circuit 13outputs the over-voltage protection signal OVP having a high level, thecontrol circuit 12 controls, based on the over-voltage protection signalOVP, the single pulse generator 181 to output a positive pulse signal.The positive pulse signal is used to control the second switchtransistor MN0 to be turned on, and the positive pulse signal isconverted to a negative pulse signal via the phase inverter 183. Thenegative pulse signal is used to control the third switch transistor MN1to be turned off, so as to control the fourth switch transistor MP0 tobe in the charging phase.

The control circuit 12 controls the single pulse generator 181 togenerate a set pulse signal by using a control signal OVP_CTRL.

In the open-loop charge pump according to the embodiment of the presentdisclosure, in a case that the over-voltage protection circuit 13outputs the over-voltage protection signal OVP having a low level, thecontrol circuit 12 controls, based on the over-voltage protection signalOVP, the single pulse generator 181 to output a negative pulse signal.The negative pulse signal is used to control the second switchtransistor MN0 to be turned off, and the negative pulse signal isconverted to a positive pulse signal via the phase inverter 183. Thepositive pulse signal is used to control the third switch transistor MN1to be turned on such that the current source 182 provides acurrent-limiting current for the current mirror, so as to decrease thepeak current.

In the embodiment shown in FIG. 4, the over-voltage protection circuit13 compares the detection voltage Vovp outputted by the output voltagedetection circuit 14 with the reference voltage Vref. When the outputvoltage VOUT exceeds the upper threshold voltage V_(OVPIN), thegenerated over-voltage protection signal OVP is a high level. Theover-voltage protection signal OVP is imputed to the control circuit 12and the single pulse generator 181 in the peak current limiting controlcircuit 18, and a signal OVP_IL and an inversion signal OVP_ILN of thesignal OVP_IL are generated, which are shown in FIG. 5. FIG. 5 is aschematic diagram showing waveforms of input and output signals of asingle pulse generator. The signal OVP_IL and the signal OVP_ILN arerespectively used to control the second switch transistor MN0 and thethird switch transistor MN1. In a case that the open-loop charge pumpoperates in the over-voltage protection mode, the over-voltageprotection signal OVP is a high level. In this case, the signal OVP_ILis a high level, the signal OVP_ILN is a low level, and the secondswitch transistor MN0 is turned on. The switch transistor in the boostcircuit 11 is in a charging phase under control of the boost controlsignal Vcon, and does not supply power to the output load Rout. In thiscase, a load current is provided by the output capacitor Cout, and theoutput voltage VOUT is gradually decreased. When the output voltage VOUTis decreased to the lower threshold voltage V_(OVPOUT) after a presettime period, the over-voltage protection signal changes to a low level,and a negative pulse signal OVP_IL and a positive pulse signal OVP_ILNhaving signal widths of Δt are generated by the single pulse generator181. During the time period of Δt, the second switch transistor MN0 isturned off, the third switch transistor MN1 is turned on, and acurrent-limiting current Ilim_ref flows to the current mirror formed bythe first switch transistor MPlim and the fourth switch transistor MP0.A current-limiting current limit flows from the fourth switch transistorMP0 after the current-limiting current Ilim_ref is mirrored by K times,which decreases a peak amplitude of the output voltage VOUT comparedwith the case shown in FIG. 1 that the peak current I_(PEAK) flows fromthe open-loop charge pump.

For example, it is assumed that Δt is set as 0.5 μs, and a capacitanceCout of the output capacitor Cout is set as 5 μF. In the open-loopcharge pump shown in FIG. 1, when the over-voltage protection signal OVPis changed from a high level to a low level, the peak current I_(PEAK),which is up to 2 A, flows from the switch transistor in the boostcircuit 11 to the output capacitor Cout. In this case, the peak voltageof the output capacitor Cout is expressed as:ΔV _(PEAK1) =I _(PEAK) *Δt/Cout=200 mV

If the load current Iload is 5 mA, a fall time of the output voltageVOUT is expressed as:Tf1=ΔV _(PEAK1) *Cout/Iload=0.2 ms

Therefore, a frequency of the over-voltage protection signal OVP isexpressed as fovp1=1/Tf1=5 KHz. The frequency is in the audio frequencyrange (from 20 Hz to 20 KHz), which results in the music noise and thepoor audio quality, affecting the hearing effect.

In the open-loop charge pump shown in FIG. 4, when the over-voltageprotection signal OVP is changed from a high level to a low level, alimited current Ilimit, such as 300 mA, flows from the switch transistorin the boost circuit 11 to the output capacitor Cout. In this case, thepeak voltage of the output capacitor Cout is expressed as:ΔV _(PEAK2) =Ilimit*Δt/Cout=30 mV

If the load current Road is 5 mA, a fall time of the output voltage VOUTis expressed as:Tf2=ΔV _(PEAK2) *Cout/Iload=0.03 ms

Therefore, a frequency of the over-voltage protection signal OVP isexpressed as fovp2=1/Tf2=33.3 KHz. The frequency is out of the audiofrequency range (from 20 Hz to 20 KHz), which does not affect the audioquality and the hearing effect.

Reference is made to FIG. 6, which is a timing diagram illustratingcomparison between ripple frequencies of output voltages of theopen-loop charge pumps shown in FIG. 1 and FIG. 4. A timing diagram of avoltage signal of the open-loop charge pump shown in FIG. 1 is shown inthe upper half of FIG. 6, and a timing diagram of a voltage signal ofthe open-loop charge pump shown in FIG. 4 is shown in the lower half ofFIG. 6. The ripple frequency may be further increased, as long as alower current-limiting current Ilimit is set. Therefore, it can be seenfrom output waveforms of the output voltage VOUT, with the open-loopcharge pump for increasing a ripple frequency of an output voltageaccording to the embodiment of the present disclosure, the ripplefrequency is increased to a frequency being out of the audio frequencyrange, thereby improving the audio quality and the hearing effect.

Based on the above description of the disclosed embodiments, thoseskilled in the art can implement or carry out the present disclosure. Itis obvious for those skilled in the art to make many modifications tothese embodiments. The general principle defined herein may be appliedto other embodiments without departing from the spirit or scope of thepresent disclosure. Therefore, the present disclosure is not limited tothe embodiments illustrated herein, but should be defined by the widestscope consistent with the principle and novel features disclosed herein.

The invention claimed is:
 1. An open-loop charge pump, comprising: acontrol circuit comprising a first input terminal and an outputterminal; a boost circuit comprising a control terminal and a loadterminal; an output voltage detection circuit; an over-voltageprotection circuit; a peak current limiting control circuit; and a clockcircuit, wherein the load terminal of the boost circuit is connected toa load circuit comprising an output capacitor and an output load, andthe load terminal of the boost circuit is connected to the first inputterminal of the control circuit via the output voltage detection circuitand the over-voltage protection circuit, wherein the output voltagedetection circuit is configured to output a detection voltage based onan output voltage at the load terminal, and the over-voltage protectioncircuit is configured to output an over-voltage protection signal basedon a reference voltage and the detection voltage; the output terminal ofthe control circuit is connected to the control terminal of the boostcircuit via the peak current limiting control circuit, and the controlcircuit drives the peak current limiting control circuit based on theover-voltage protection signal to control the boost circuit to be in acharging phase continuously or in a normal operation mode; in a casethat the output voltage is higher than an upper threshold voltage, theboost circuit is continuously in the charging phase and does not supplypower to the output load, and the output capacitor is discharged tosupply power to the output load; and in a case that the output voltageis lower than a lower threshold voltage, the boost circuit is in thenormal operation mode, wherein in a time period of the normal operationmode, the boost circuit switches between a discharging phase and thecharging phase based on a frequency of the clock circuit, and when theboost circuit switches from the charging phase to the discharging phase,the peak current limiting control circuit decreases a peak currentoutputted from the boost circuit to increase a ripple frequency of theoutput voltage.
 2. The open-loop charge pump according to claim 1,wherein the peak current limiting control circuit comprises: a firstswitch transistor; a second switch transistor; a third switchtransistor; and a single pulse generator, wherein a gate of the firstswitch transistor is connected to the control terminal of the boostcircuit, a first electrode of the first switch transistor is configuredto receive a power supply voltage, and a second electrode of the firstswitch transistor is connected to the gate of the first switchtransistor; a gate of the second switch transistor is connected to anoutput terminal of the single pulse generator, a first electrode of thesecond switch transistor is grounded, and a second electrode of thesecond switch transistor is connected to the second electrode of thefirst switch transistor; a gate of the third switch transistor isconnected to the output terminal of the single pulse generator via aphase inverter, a first electrode of the third switch transistor isgrounded, and a second electrode of the third switch transistor isconnected to the second electrode of the first switch transistor via acurrent source, wherein an input terminal of the phase inverter isconnected to the output terminal of the single pulse generator, and anoutput terminal of the phase inverter is connected to the gate of thethird switch transistor; and an input terminal of the single pulsegenerator is connected to the output terminal of the control circuit. 3.The open-loop charge pump according to claim 2, wherein the first switchtransistor is a P-channel Metal Oxide Semiconductor (PMOS) transistor,and both the second switch transistor and the third switch transistorare N-channel Metal Oxide Semiconductor (NMOS) transistors.
 4. Theopen-loop charge pump according to claim 2, wherein the boost circuitcomprises: a fourth switch transistor; and a function circuit, wherein agate of the fourth switch transistor is the control terminal of theboost circuit, a first electrode of the fourth switch transistor isconfigured to receive the power supply voltage, and a second electrodeof the fourth switch transistor is connected to the load circuit via thefunction circuit; and the fourth switch transistor and the first switchtransistor form a current mirror to limit an output current of thesecond electrode of the fourth switch transistor, so as to decrease thepeak current.
 5. The open-loop charge pump according to claim 4, whereinthe fourth switch transistor is a PMOS transistor.
 6. The open-loopcharge pump according to claim 4, wherein the over-voltage protectioncircuit outputs the over-voltage protection signal having a high levelin a case that the output voltage is higher than the upper thresholdvoltage, and outputs the over-voltage protection signal having a lowlevel in a case that the output voltage is lower than the lowerthreshold voltage.
 7. The open-loop charge pump according to claim 6,wherein in the case that the over-voltage protection circuit outputs theover-voltage protection signal having a high level, the control circuitcontrols, based on the over-voltage protection signal, the single pulsegenerator to output a positive pulse signal; and the positive pulsesignal is used to control the second switch transistor to be turned on,and the positive pulse signal is converted to a negative pulse signalvia the phase inverter, wherein the negative pulse signal is used tocontrol the third switch transistor to be turned off, so as to controlthe fourth switch transistor to be in the charging phase.
 8. Theopen-loop charge pump according to claim 6, wherein in the case that theover-voltage protection circuit outputs the over-voltage protectionsignal having a low level, the control circuit controls, based on theover-voltage protection signal, the single pulse generator to output anegative pulse signal; and the negative pulse signal is used to controlthe second switch transistor to be turned off, and the negative pulsesignal is converted to a positive pulse signal via the phase inverter,wherein the positive pulse signal is used to control the third switchtransistor to be turned on such that the current source provides acurrent-limiting current for the current mirror, so as to decrease thepeak current.
 9. The open-loop charge pump according to claim 2, whereinthe single pulse generator is configured to output a pulse signal havinga signal width of 0.5 μs.